The present invention relates to an optical pick-up apparatus by which at least one of the recording and reproducing of the information can be conducted for 3 kinds of the optical information recording media whose standards (recording density) are different, objective lens which can be used for the optical pick-up apparatus, substrate thickness difference correcting means, chromatic aberration correcting means, substrate thickness error correcting means, aperture regulating/changing means, optical element and a recording•reproducing apparatus using this optical pick-up apparatus.
Conventionally, the optical disk is, as well known in CD (Compact Disk) or DVD (Digital Versatile Disc), widely used for the storage of the digital data such as the accumulation of the music information and image information, or storage of the computer data. In such a situation, recently, together with the arrival of an information-oriented society, the increase of capacity of these optical disks is intensely required.
In the optical disk, to increase the storage capacity per a unit area (recording density), it can be realized by reducing the spot diameter obtained from the optical pick-up optical system. Because the minimum diameter of the spot, as well known, is proportional to λ/NA (where, λ is a wavelength of the light source, and NA is a numerical aperture of the optical system), in order to reduce the spot diameter, to reduce the wavelength of the light source used in the optical pick-up apparatus, and to increase the numerical aperture of the objective lens arranged opposite to the optical information recording medium in the optical system, are effective.
In them, relating to the reduction of the wavelength of the light source, it can be said that the study of the blue violet semiconductor laser generating the light of the wavelength of about 400 nm, or SHG blue violet laser is advanced, and the practical use of them is in a short time. When this short wavelength light source is used, even when the objective lens of NA 0.65 which is the same as DVD is used, onto the 12 cm diameter optical disk which is the same as the DVD, the recording of the information of about 15 GB can be conducted.
Further, relating to the increase of the numerical aperture of the objective lens, the study of the objective lens to realize the high numerical aperture of NA 0.85 by combining two lenses is advanced. When the above short wavelength light source and the high numerical aperture objective lens of NA 0.85 are used by combining them, onto the 12 cm diameter optical disk, the recording of the information of about 25 GB can be conducted, thereby, the more increase of the density can be attained.
In such a background, the study and development of the optical pick-up system to record and reproduce the high density next generation optical disk (hereinafter, in the present specification, called “high density DVD”) is advanced, and in the optical pick-up system, it is required that 3 kinds of optical disks whose standards (recording density) are different, such as the high density DVD, DVD and CD, are compatibly recorded and reproduced.
As a means to attain the compatibility, it can be considered that the objective lenses corresponding to the standard of respective optical disks are mechanically switched, or the optical pick-up apparatus corresponding to the standard of respective optical disks are mounted, however, in this case, the switching mechanism of the objective lens or a plurality of optical pick-up apparatus are necessary, resulting in the increase of the size of the optical pick-up system, the increase of the number of parts, and the increase of the production cost. Accordingly, in the optical pick-up system for the high density DVD in which the compatibility of the DVD and CD is required, for the purpose of reduction of the size of the optical pick-up system, reduction of the number of parts, and reduction of the cost, it can be said that the most preferable one is to conduct the recording and reproducing onto the 3 kinds of optical disks whose standards are different, by using the common objective lens.
Problems in the case where the optical pick-up system by which the recording and reproducing are conducted by using the common objective lens onto the 3 kinds of optical disks whose standards are different, is realized, will be described below.
(1) To Secure the Distance (Working Distance) of the Objective Lens to CD when the CD is Recorded and Reproduced.
This is a problem which is conspicuous when the NA of the objective lens is more increased than that of the DVD (NA 0.6 to 0.65). In the high density DVD, when the NA of the objective lens is increased, because, when the protective substrate (called also transparent substrate or protective layer) thickness is remained 0.6 mm which is the same as the DVD, the coma generated by the skew (inclination or warpage) of the optical disk is increased, it is necessary that the protective substrate thickness is more reduced than 0.6 mm, and the margin to the skew of the optical disk is secured. As the high NA objective lens for such a high density DVD, the objective lens composed of 2 lenses is written in Tokkaihei No. 10-123410.
On the other hand, because the working distance of the objective lens of 2-group composition is shorter than that of the conventional objective lens of 1 group composition, it is very difficult to secure the sufficient working distance when the CD is recorded and reproduced. Because the NA of the CD is NA 0.45 to 0.5, which is smaller than that of the high density DVD or DVD, the tolerance to the production error of the protective substrate thickness is not so severe, therefore, the fluctuation by the individual difference of the protective substrate thickness is large. Accordingly, in order to prevent the breakage of the data by the collision of the CD and objective lens, it is necessary to sufficiently secure the working distance when the CD is recorded and reproduced, and from this point of view, in the optical pick-up system by which recording and reproducing are conducted by using the common objective lens onto 3 kinds of optical disks whose standards are different, it can be said that the objective lens of 2-group composition is undesirable.
(2) Spherical Aberration Generated Due to the Difference of the Protective Substrate Thickness
In the CD (protective substrate thickness 1.2 mm) and DVD (protective substrate thickness 0.6 mm), there is a difference of 0.6 mm in the protective substrate thickness. Accordingly, in the optical pick-up system by which recording and reproducing are conducted by using the common objective lens onto 3 kinds of optical disks whose standards are different, it is necessary to provide a means to correct, at least, the spherical aberration due to the difference of the protective substrate thickness of the CD and DVD. Further, when the protective substrate thickness of the high density DVD is more reduced than 0.6 mm by the increase of the NA of the objective lens, it is necessary to provide a means to correct the spherical aberration by the difference of the protective substrate thickness of 3 kinds of the optical disks whose protective substrate thickness are different from each other.
(3) Spherical Aberration Generated Due to the Difference of the Wavelength
As described above, in the high density DVD, to attain the increase of the density, the blue violet light source of wavelength of about 400 nm is used. However, in the conventional refraction type objective lens, because, by the dispersion of the lens material, the spherical aberration is changed, it is difficult that the light flux from the light source having the different wavelength is converged in almost no-aberration onto the information recording surfaces of respective optical disks. Therefore, in order to attain the compatibility of the high density DVD (wavelength 400 nm), DVD (wavelength 650 nm), and CD (wavelength 780 nm), any means is necessary to correct the spherical aberration due to the difference of the wavelength.
However, in the optical pick-up apparatus, by the reduction of the wavelength of the light source and the increase of the NA of the objective lens, the problems which will be described below, are actualized.
The first problem is a chromatic aberration generated in the objective lens when the blue violet semiconductor laser generating the short wavelength of about 400 nm is used as the light source.
In the optical pick-up apparatus, the laser light emitted from the semiconductor laser is generally a single wavelength (single mode), and therefore, it is considered that the chromatic aberration of the objective lens has no problem, however, actually, the mode hopping in which the central wavelength is changed instantaneously by several nms due to the temperature change or output change, is produced. Because the mode hopping is a wavelength change caused instantaneously in such a manner that the focusing mechanism of the objective lens can not follow it, when the chromatic aberration of the objective lens is not corrected, the de-focus component corresponding to the movement amount of the image formation position is added, thereby the light converging performance of the objective lens is deteriorated.
The dispersion of general lens material used in the objective lens, in 600 nm to 800 nm which are wavelength region of the infrared semiconductor laser or the red semiconductor laser, is not so large, therefore, in the CD or DVD, the deterioration of the light converging performance by the mode hopping is not problem.
On the one hand, in the vicinity of 400 nm which is the wavelength region of the blue violet semiconductor laser, because the dispersion of the lens material is very large, even when the wavelength change is mere several nms, the image formation position of the objective lens is largely shifted. Therefore, in the high density DVD, when the blue violet semiconductor laser is used as the light source, the light converging performance of the objective lens is largely changed when the light source occurs mode hopping, thereby, there is a possibility that the stable recording or reproducing can not be conducted.
The second problem is a problem of the spherical aberration generated by various error factors when NA of the objective lens is increased.
In the optical pick-up apparatus, the spherical aberration generated due to the production error of the protective substrate thickness of the optical disk is proportional to the fourth power of the NA of the objective lens. In order to suppress this spherical aberration within the allowance, it is necessary that the tolerance to the production error of the protective substrate thickness of the optical disk is made several μm, however, in this case, the yield of the production of the optical disk is lowered, and there is a possibility that the mass production can not be attained. Accordingly, in the high density DVD, when the NA of the objective lens is increased, it is necessary to provide a means to correct the spherical aberration generated due to the production error of the protective substrate thickness of the optical disk.
Further, in the optical pick-up apparatus, as the objective lens, many plastic lenses are used because they are advantageous for the mass production. However, in the temperature change of the refractive index, it is well known that the plastic lens is about 2 figures larger than that of the glass lens.
When the environmental temperature of the objective lens formed of the plastic material rises and the refractive index of the objective lens is changed, in the objective lens, the spherical aberration is deteriorated. The deterioration amount of the spherical aberration due to the refractive index change is proportional to forth power of the NA of the objective lens, therefore, in the high density DVD for which the high NA objective lens is used, when the plastic lens is used for the objective lens, because the usable temperature region is too narrow, it is necessary to provide a means to correct the spherical aberration generated due to the refractive index change.